1. Field of the Invention
The present invention relates generally to systems and methods for molding composite parts. More particularly, the invention is directed to systems and methods that apply pressure to the composite structure during the molding step.
2. Description of Related Art
Composite materials are used extensively in the aerospace and marine industries, the wind energy turbine industry and in other situations where high strength and relatively light weight are desired. Composites typically include fibers and polymer resin as the two principal elements. A wide range of fiber types has been used in composites. Glass, graphite, carbon and ceramic fiber are common. The fibers can be chopped, randomly oriented, unidirectional in orientation or woven into fabric. The fibers used in composite materials have diameters that range from extremely small to relatively large. Although it is possible to make composites using large diameter fibers, the more common practice is to take thousands of fibers having extremely small diameters and form them into individual bundles known as tows. These multi-fiber tows are much stronger and more flexible than single fibers having the same overall diameter. The tows can be woven into fabric in the same manner a conventional yarns. Alternatively, the tows are arranged in parallel to provide a unidirectional fiber orientation or they can be randomly oriented. The ways in which the polymer resin is infused or impregnated into the complex fiber structure and the ways in which the resulting resin/fiber structure is cured are important considerations in the molding of composite parts.
There are a number of ways to combine the polymer resin with the fibers and there are a number of ways to cure or mold the resulting composite body to form the final composite part. One approach, which has been in use for years, is to manually impregnate the fibers with activated resin in-situ on a mold or other support structure. Depending on the particular resin system, heat may or may not then be applied to cure the resulting “lay-up”. This type of manual lay-up procedure is popular because it is simple, requires little, if any, special tools and can be used to make large parts, such as boat hulls. However, it is difficult to accurately control the amount of resin that is applied to the fibers and to insure that the resin is being uniformly impregnated into the fiber tows. In addition, the amounts of curing agent and other additives that are added to the resin may vary between lay-ups.
In order to avoid the above problems, it has been common practice to form a prefabricated lay-up (prepreg) that includes the fiber and resin matrix (resin, curing agents and any additives). The prepreg is made under manufacturing conditions that allow the amount and distribution of resin matrix within the prepreg to be carefully controlled. Once formed, the prepreg may be applied to a mold or other support surface in the same manner as a conventional manual lay-up. In general, prepregs are not used immediately after they are formed. Instead, they usually are stored for use at a later time. Another popular way to combine the polymer resin and fibers is to use a vacuum to infuse the polymer resin into the fiber structure. Such vacuum infusion methods typically use a vacuum bag to surround the fiber structure during resin infusion.
There are also a number of ways to cure or mold the combined polymer resin/fiber composite body. A common practice is to heat the composite body while at the same time applying pressure to the body. This is typically accomplished using an autoclave. The use of positive pressure during molding provides many benefits including: reducing voids in the final composite part, providing complete resin infiltration of thick fiber structures, diminishing resin rich areas that are more susceptible to delamination, allowing for the use of high fiber volume fractions, allowing for the use of high viscosity resins and generally improving mechanical properties.
Large and/or thick solid composite parts, such as wind energy turbine blades, ship hulls, bridge decks and similar large-scale components are not practical to process in an autoclave. Due to the size and process limitations, such large scale structures are often manufactured using vacuum infusion methods or vacuum bag only curing for pre-preg lay-ups. Processing large parts using infusion methods does present some problems. For example, it is difficult to control the resin distribution during curing because the vertical sections in large structures may experience “vertical sag”. Vertical sag occurs when the uncured resin flows due to gravity during extended-time lay-ups or during the cure cycle when the viscosity of the resin typically decreases. This creates uneven thickness and differences in resin content through the composite structure. Vacuum only processing of prepreg lay-ups can produce better results in terms of resin content homogeneity and a higher fiber volume fraction. However, vacuum-only processing tends to produce large parts that have higher void content due to air entrapment between prepreg layers. Less than 2% void content in composite parts having 55% fiber volume is possible with vacuum bag-only processing. However, the large scale laminates made using vacuum-only processing still exhibit higher void content than structures made using an autoclave or other system that applies positive pressure, in addition to atmospheric pressure, during the curing step.
There are a number of systems that are designed to provide an alternative to the conventional autoclave for applying positive pressure and heat to a composite structure during cure. Such systems employ a variety of solid and flexible mold components to provide application of positive pressure and heat to the composite structure during curing. One example is referred to as the “Quickstep” process where the composite material is paced in a mold and sealed with a flexible sheet or matched mold. The tooling and part are then place inside a sealed tank with heat transfer fluid. The heat transfer fluid is used to apply and pressure to the composite material during curing.
Another example of a pressure application system is described in U.S. Pat. No. 6,435,242 where a positive pressure is applied by a bladder system that is secured to the mold via a vacuum seal. Other examples of pressure application systems that utilize combinations of flexible bladders and rigid components include: U.S. Pat. Nos. 6,537,483; 6,746,737; 6,692,681; 6,666,651; 6,596,121; 6,319,346; and German Patent DE 10150659. A closed-mold resin infusion process has also been developed where positive pressure is applied using a rigid closed mold that includes an internal flexible sheet on one surface (See “Investigation of a Two-Stage Injector Process to Reduce The Effects of In-Plane Resin Flow”, Larsen et. al., Montana State University, AIAA-2002-0026).
Even though the above systems are well suited for their intended purpose, there is a continuing need to develop new systems that can be used as an alternative to the autoclave for applying pressure and heat, if required, to composite structures during the curing process. This continuing need is especially present with respect to the molding of large composite parts, such as wind energy turbine blades and large aerospace components.